In this paper, numerical simulation of projectile’s underwater motion process with tail-slaps was studied based on CEL (coupled Eulerian-Lagrangian analysis) method. The stability principle of projectile’s motion with tail-slaps was studied. Some parameters, such as initial angle of attack when tail-slaps occurred, maximum angle of attack, and instability critical angle of attack were used to characterize the projectile’s tail-slap. The relationship between stability and angle of attack was revealed. The location change of pressure center caused by projectile’s tail-slap was studied, too. The results showed that the static stability margin of projectile was enough to meet its stability requirements. Finally, three constraint criteria for the motion stability of projectile, including the requirement for the size of cavity, the relative location of center of pressure and center of mass, and the static stability margin of projectile, were obtained. The location and area of tail-slap can be used to control the static stability margin of projectile, some suggestions for the design of projectile’s shape were proposed, too.

At present, the International Maritime Organization (IMO) has issued the final guidelines of the direct stability assessment of surf-riding/broaching for the second generation intact stability criteria, and how to accurately and efficiently predict the surf-riding/broaching is a key problem to be solved for the direct stability assessment of surf-riding/broaching. So a surge-sway-heave-roll-pitch-yaw coupled mathematical model (6-DOF) is established in this paper. Firstly, the heave and pitch motions are considered in the surge-sway-roll-yaw maneuvering mathematical model, and the amplitudes and phases of heave and pitch motions are calculated by a strip method using an enhanced integrating method, which can solve the problem of divergence resulting from direct seakeeping calculation in time-domain for high speed vessel in stern-quartering waves. Secondly, the Froude-Krylov forces and diffraction forces are calculated by integrating the wave pressure up to the mean wave surface. At the same time, nonlinear hydrodynamic derivations, heel-induced hydrodynamic forces and nonlinear roll damping are considered in the mathematical model. The hydrodynamic lift forces due to the coexistence of wave particle velocity and ship forward velocity are taken into account in the propeller thrust and rudder force model. And the real-time emersion of double rudders in waves is considered in the rudder force model. Finally, a tumblehome ship with twin propellers and double rudders is utilized to study surf-riding/broaching in stern-quartering waves, and the effect of initial relative position of the ship to waves on predicting surf-riding/broaching motion is investigated. The computation results show that the established 6-DOF mathematical model has enough accuracy to be used for the direct stability assessment of the surf-riding/broaching failure mode.

The elimination of long period waves has always been a difficult problem in coastal engineering that needs to be solved. Plate breakwater has a certain effect on long period waves, due to its convenience in construction, low cost and wide application range. Further research of which is of great significance. A numerical wave making flume is established based on Flow-3D in this paper, the optimal placement scheme and dimensions of the flat plate array breakwater are presented with the relative plate length L₀/L, relative plate spacing j/L, relative plate thickness d/H and number of plates N identified as research parameters. Simulation results show that, when the relative plate thickness d/H=0.1, plate number N=4, relative plate spacing j₁/L=0.3, j₂/L=0.1, relative plate length L₀/L=0.6, the wave suppression effect of non-uniform four-plate array breakwater is the best. It is shown that the arrangement form has good wave elimination for long period waves with period less than T=8 s (prototype period T=25.28 s).

Dolphins and other toothed cetaceans have high-speed swimming ability and are important research objects for underwater biomimetic drag reduction. In this paper, based on the characteristics of dolphin skin structure and adaptive deformation, an engineering-scale bionic study was carried out, a wall deformation motion control equation with the normal velocity of the boundary layer of the wall surface as the input signal was designed, and the drag reduction performance of the deformed wall surface was simulated by the dynamic mesh technology. The results show that the optimal frictional resistance reduction rate of the deformed wall surface is 19.24% and the optimal total resistance reduction rate is 6.4% at a flow rate of 0.5 to 10 m/s. Application of the biomimetic control results in the increase of the thickness of the turbulent boundary layer of the wall surface, reduction of both the surface friction resistance and the turbulence kinetic energy of the flow field. The bionic deformation wall was applied to the surface of the non-attached SUBOFF submarine model to carry out drag reduction design. In the speed range of 3.045-8.231 m/s, the total drag reduction rate greater than 8.0% was obtained.

A sea-launching ship will have complex nonlinear motion response under the environmental loads and launch impact load, which has a vital effect on the safety of rocket launching process. Based on the three-dimensional potential flow theory, this paper focuses on the motion responses of a sea-launching ship under the action of wind, waves, and impact loads in different sea states. And the effects of the dynamic position system and the launch ignition time on the motion of the launch ship were investigated. The results show that the launch impact load has a great influence on the pitch motion of the ship, especially when the wind/wave direction is parallel to the ship direction. The existence of dynamic position system will increase the roll of the launch ship. And the launch ignition time can be selected at the moment when the pitch motion of the launch ship is about to reach the extreme point, so as to reduce the impact of the launch ship's motion on the rocket attitude during taking-off.

Based on the OpenFOAM, a hybrid model coupling fully nonlinear potential flow theory (FNPT) with viscous flow method, and the propeller-rudder mode, the turn and zigzag maneuvers of a single-screw ship in waves were simulated. The FNPT was used to simulate the wave tank in the external field, while the viscous flow method was used to simulate the interaction between waves and ships in interal field. Then, the 6DOF ship maneuvers in waves was simulated. The KVLCC2 model was selected for simulation, and the method was validated by the tank test, and the maneuvers in beam waves were simulated. By various wavelengths, the effect of wavelength on the ship maneuvering performance in beam waves was investigated.

Tank sloshing is a common phenomenon in ship navigation, which not only affects the stability of a ship, but also poses a threat to the marine environment and human life. How to reduce the amplitude of tank sloshing has always been a key research problem in ocean engineering. In this paper, based on the improved moving particle semi-implicit method, BM-MPS method, the damping effect of vertical open-hole separator under lateral excitation was simulated and studied, and the influence of porosity and lateral amplitude of the separator on resonance period and impact duration curve morphology was discussed. The results show that the size of porosity has an obvious effect on the resonance period. The porosity of 15% is the point where the maximum impact pressure is the minimum when the resonance is reached, and the resonance period is in the transition period of obvious transformation. The impact duration curve is also closely related to porosity. With the increase of porosity, the impact duration curve changes from no peak to single peak and then to double peak. In addition, the amplitude of excitation is also one of the factors affecting the resonance period.

The effect of riblet wall on the two-phase turbulent boundary layer containing 355 μm polystyrene particles was investigated by using particle image velocimetry (PIV). The effects of riblet wall on the characteristics of two-phase turbulent boundary layer were analyzed by comparing the average velocity profile, turbulence and Reynolds shear stress of the flow over the riblet and smooth surface. The spatial multi-scale local average structure function and λ_ci vorticity identification criterion of hairpin vortex head (clockwise spanwise vortex) were used together to accurately identify the vortex center and extract the spatial topology of the surrounding fluctuating velocity and streamline. The numerical law of the forward vortices at different wall-normal heights was calculated. The results show that, compared with the smooth surface, the buffer layer of the average velocity profile of the riblet wall rises partly, the logarithmic region shifts outward, and the friction velocity and frictional shear stress of the riblet wall decrease, resluting in a drag reduction of 4.86%. At the same normal height, compared with the smooth wall, the clockwise vortexes detected on the riblet wall have smaller dip angles and fewer numbers, and the streamwise fluctuating velocity around the riblet wall is weak, showing that in two-phase flow, the riblet wall can weaken the intensity and finally decreases the drag.

Based on the analysis of the lubrication performance of the eccentric stern bearing, an elasto-hydrodynamic coupling lubrication model for the local wear and stiffness of the stern bearing was established. The joint program of finite difference method and finite element method was compiled to solve the elastic deformation of the bearing, and the mass conservation boundary condition was used to replace the Reynolds boundary condition. The effects of local wear depth, bearing elastic modulus and other factors on the hydrodynamic pressure, liquid film thickness, cavitation area and friction law of the bearing were discussed in detail. The results show that when the local wear depth of rigid body bearing is lower than the threshold value, it is beneficial for bearing lubrication. When the local wear depth exceeds the threshold value, the maximum hydrodynamic pressure, friction force and cavitation area increase significantly. The influence of bearing elastic deformation on the calculation results cannot be ignored. Elastic deformation and local wear exist at the same time, and the change law is basically consistent with the change trend of local wear of rigid body bearings.

In order to predict the critical buckling load of a filament winding thick composite cylindrical shell under hydrostatic pressure, the buckling governing equation of the thick cylindrical shell under hydrostatic pressure was obtained based on the nonlinear Sander theory, as well as the deformation geometry equation of the cylindrical shell and the constitutive relation of the filament-wound layer. An analytical method for predicting the critical buckling pressure of thick composite cylindrical shells under hydrostatic pressure was proposed by solving the governing equation. Then, critical buckling load of the thick shell with different filament-wound types and angles were calculated with FEM and compared with analytical results for verifying the accuracy and high efficiency of the analytical method. The influence of key parameters such as geometrical and material design on the critical buckling load of thick cylindrical shells was investigated based on the analytical method.

Ocean thermal energy conversion is one of the research hotspots of marine renewable energy in recent years. Free-hanging water intake pipes are the key structure to extract deep cold seawater. At present, the vortex-induced vibration (VIV) response characteristics of free-hanging pipes in deep sea currents are not clear yet. In this paper, model tests of a free-hanging pipe under uniform flow were carried out, and the strain response of vortex-induced vibration was measured by the fiber Bragg grating strain sensor. The amplitude and frequency characteristics of the free-hanging pipe were investigated by modal analysis and wavelet transform data processing methods. It is revealed that the maximum amplitude of the VIV displacement response of the free-hanging pipe under the uniform flow mainly occurs at the bottom. The dominant frequencies in inline (IL) direction is basically two times that in cross flow (CF) direction. However, in the conditions where the modal transition occurs, the dominant frequencies in IL and CF directions are the same, accompanied by obvious "traveling wave", "multi-frequency response" and "time-sharing" phenomenon. In addition, the Strouhal number of the overhanging pipe model in CF and IL under uniform sea currents are 0.15 and 0.30, which are slightly smaller than the results of flexible risers hinged at both ends. This value may serve as the parameter input for the vortex-induced vibration prediction of free-hanging pipes.

The cyclic void growth model (CVGM) based on microscopic fracture mechanism is an effective method to analyze and predict ultra-low cycle fatigue (ULCF) fracture. In this method, the void growth index and damage degradation parameters are important parameters to control the crack propagation process. Due to the influence of production technology, the void growth index and damage degradation parameters of different batches of steels are often unfixed, which leads to the insufficient accuracy of the ultra-low cycle fatigue fracture analysis. In order to solve this problem, smooth round bar, smooth notch round bar and notch samples were used to carry out experiments, the ultra-low cycle fatigue characteristics of steel were studied, and the cracking mechanism and damage evolution law of the samples were explored. Secondly, the VUSDFLD program based on the cyclic void growth model was written, and the finite element analysis was carried out based on the results of test to calibrate the void growth index and damage degradation parameters. Finally, the ultra-low cycle fatigue fracture of notched samples was studied, the crack initiation and crack propagation rates were analyzed, and the ultra-low cycle fatigue fracture life of the samples was predicted. The results show that the fracture process of ultra-low cycle fatigue matches well with the experimental results, which is suitable for the prediction of ultra-low cycle fatigue fracture life.

Fatigue problem as a common failure form in the engineering field has been widely concerned. The fatigue damage-crack size can be obtained by the fatigue analysis method based on fracture mechanics, but the calculation is relatively complicated. In this paper, aiming at the spectrum analysis based fatigue analysis of ocean engineering structures, the stress intensity factor (SIF) spectrum under random loading conditions of the same hot spot through genetic algorithm wavelet neural network (GAWNN) was established, and the network training with the SIF obtained from finite element analysis was conducted. The results show that the model can predict the SIF spectra under random loading conditions well. The method proposed in this paper can considerably reduce the repetitive finite element calculation and provide a reference for the fatigue life prediction of engineering structures under random load conditions by applying crack propagation method. Finally, combined with the unique crack growth rate curve model, the rapid prediction of crack growth under random loading conditions was realized.

In order to effectively reduce the ‘window effect’ in the sound field reconstruction process of near-field acoustic holography (NAH) and accurately measure and locate the surface sound source of a submarine by using the limited measuring aperture, a patch NAH method based on two-level iteration was proposed. Firstly, patch NAH based on orthogonal spherical waves was improved, and a patch NAH method based on two-level iteration was proposed. Then, the reconstruction results of the two NAH methods were simulated and compared, and the influence of extrapolation error and extended measurement points on the reconstruction accuracy was studied. Finally, the experimental study of patch NAH based on two-level iteration was carried out in an anechoic tank using a small aperture holographic measuring surface. The results show that the patch NAH based on two-level iteration proposed in this paper can greatly reduce the ‘window effect’ and ‘aperture repetition effect’ errors caused by the limited measurement aperture, which verifies the advantages of the method and provides an important reference for NAH precise measurement of large size sound sources.

Sandwich plates, as a new structure, have the advantages of strong design, light weight, high specific stiffness, good performance of vibration reduction and sound insulation, and have been used in various engineering structures. However, there are many types of sandwich structures, and it is difficult to theoretically analyze the sound insulation performance of complex sandwich structures. New approximate analysis methods are urgently needed. In this paper, based on the equivalent method of spring, torsion spring and concentrated mass, the sound-vibration coupling control equation of a C-type sandwich panel was established for the marine C-type sandwich panel. Using the spatial harmonic expansion method, the expression of the sound transmission loss of the C-shaped sandwich structure was derived. The influence of the system parameters of the sandwich panel on the sound insulation performance of the structure was discussed. The results show that the material properties and the thickness of the sandwich panel structure have greater influence on the sound insulation performance of the C-type sandwich panel structure. The finite element simulation was used to verify the validity of the theoretical analysis.